Initiation of fuel combustion, particularly for compression-type internal combustion engines, is a well-developed art which has its origin in the Otto-cycle Spark Ignition (SI) engine that was developed in the late 1800's. During the past century, the internal combustion (IC) engine has undergone considerable improvement in its design and performance. Along with basic IC engine development have come considerable technological improvements in the associated ignition system.
The earliest ignition systems employed a high voltage magneto. The magneto was gradually replaced during the 1920's by a battery-based induction coil system which utilized mechanical breaker points as a current-interrupt switch. The coil ignition (CI), invented by Charles Kettering, became the standard for automotive applications and maintained that status for several decades with remarkably little change in design or operation.
The advent of reliable semiconductor switching devices, commencing approximately 30 years ago, introduced technology which led to the gradual elimination of performance limitations and maintenance problems associated with mechanical breaker points. Transistor-assisted-contact (TAC) systems were devised in which a transistor switch relieved the mechanical breaker points of the burden of carrying high current flow. More recently, mechanical breaker points have been entirely replaced by "breakerless" timing circuitry and ignition systems based exclusively on semiconductor switching technology. Recent efforts have also been made to eliminate the conventional mechanical rotor system for high voltage ignition pulse distribution.
The availability of fast switching power transistors and thyristor devices (e.g., silicon controlled rectifiers) has given rise during the last few decades to a variety of capacitor discharge ignition (CDI) systems. In contrast to the inherently slower (typically 60-200 microseconds rise time) longer lasting (typically 1-2 milliseconds) output pulses characteristic of induction coil systems, CDI systems provide faster rising pulses (1-50 microseconds) at the expense of shorter overall duration (5-500 microseconds). The faster rising pulses of CDI systems are less susceptible to misfire due to spark plug fouling.
Modern conventional coil and capacitor discharge systems usually deliver between 5 to 100 millijoules (mJ) of electrical energy per pulse at peak output voltages ranging from 20,000 to 30,000 volts. The more common systems operate in the energy range of 20 to 50 mJ per pulse.
Before discussing prior art systems in more detail, it is necessary to appreciate the physical phenomena by which thermal ignition occurs. Gaseous electrical discharge typically occurs in three common phases:
(1) a breakdown phase, usually less than a few tens of nanoseconds in duration, in which current flow increases rapidly as the voltage across the discharge gap falls, PA0 (2) a transition to arc discharge of relatively high internal energy content and current density, PA0 (3) possibly followed by transition to glow discharge characterized by somewhat lower internal energy and current density.
The overall duration of an ignition system discharge, and the relative fraction of total energy deposited during the breakdown, arc, and glow phases are primarily governed by the circuit parameters of the system. The discharge circuits of conventional systems typically have high inductance, low capacitance and relatively high resistance. These high impedance systems couple only a small fraction of the discharge energy into the fuel mixture during the brief breakdown phase. CDI systems generally deliver a current pulse consisting primarily of the arc phase. Transistor-coil-ignition (TCI) systems on the other hand, emphasize a relatively quick transition from breakdown to a long duration, low current glow discharge which is accomplished by gradually releasing the energy stored in the magnetic field of the coil through a high impedance discharge circuit.
Within the last several years, the establishment of strict exhaust emission standards and a demand for better fuel efficiency have placed additional constraints on engine operation. In response to these demands, recent trends in engine design and operation have been toward promoting a faster combustion process and extending stable operation to leaner fuel mixtures.
Operating with lean or EGR (exhaust gas recycle)-diluted mixtures can achieve significant reductions in exhaust emissions while increasing thermal combustion efficiency and reducing specific fuel consumption. Conversely, lean burn is characterized by more difficult ignition and slower laminar flame velocity, which, with increasing mixture dilution, eventually leads to cycle-by-cycle (CBC) variations, incomplete combustion, and a subsequent increase in unburned hydrocarbon emissions.
Promoting a faster combustion process, on the other hand, increases engine cycle efficiency (thereby lowering specific fuel consumption), permits operation with lower octane fuels or higher compression ratios, reduces CBC variations, and allows for stable engine operation with more dilute fuel mixtures.
It is known that higher combustion efficiencies can be achieved by increasing compression ratios; at a given compression ratio, highest operating efficiency occurs under conditions of constant-volume heat addition (i.e. very rapid combustion) which corresponds to a very rapid (ideally instantaneous) combustion process. Thus, fast-burn Otto-cycle engines are theoretically capable of achieving higher overall cycle efficiency than diesel engines at a given compression ratio. In practice, however, diesel engines are generally more efficient than relatively slow burning gasoline engines due to the ability of the diesel to work at higher compression ratios. However, with faster combustion rates, the Otto-cycle engine efficiency not only increases but also permits operation at higher compression ratios. This in turn leads to further increases in efficiency which can result in Otto-cycle engine performance which more closely approaches conventional diesel engines.
Turbulence is known to be a mechanism by which the effective rate of combustion can be increased. A primary approach toward faster, leaner burn operation has involved the development of engine designs which enhance turbulence and fluid mechanical effects in the mixture within the combustion chamber.
It has been experimentally established that minimum spark ignition energy requirements correspond to fuel mixtures which are at, or somewhat rich of, the stoichiometric* ratio. This mixture range corresponds to maximum laminar flame velocity and maximum engine power output, and is the point where engines traditionally operated prior to 1970. However, as the fuel mixture becomes leaner, the minimum energy required for ignition increases dramatically. Furthermore, ignition of flowing mixtures can be more difficult than ignition of the same mixture under quiescent conditions. Consequently, the increased bulk fluid motion and turbulence which is often introduced into the fuel mixture to promote more rapid combustion adds to the demands of an ignition system which is already burdened by the difficulties of igniting a leaner mixture. In the past, it has not been possible to enhance ignition performance to satisfactorily overcome these problems. Moreover, successful engine operation with very lean mixtures** can ultimately only be achieved by the combined application of ignition enhancement measures and combustion rate increase mechanisms which offset the general slowdown in combustion kinetics that accompany mixture dilution. FNT *A stoichiometric air-to-fuel mixture contains the exact amount of air necessary to completely burn the fuel. The air-to-fuel mass ratio for octane is about 15:1. FNT **Air to fuel ratios greater (leaner) than about 20:1
As used herein, factors which promote more rapid overall combustion are termed "combustion enhancement" mechanisms while factors that promote a quicker, more probable initiation of combustion are termed "ignition enhancement" mechanisms. Ideally, it would be desirable for an ignition system to provide enhancement factors that surpass the early ignition stage and influence the entire combustion process.
Considerable controversy has existed in the past as to how ignition enhancement can best be achieved. This has been due in part to the lack of adequate theory to satisfactorily model the broad scope of complex physical and chemical processes which take place during spark discharge ignition. It has been generally accepted that the main spark ignition mechanism involves the creation of a volume of hot ionized gas (plasma ignition kernel) which envelopes a sufficient quantity of fuel mixture for a sufficient length of time to thermally initiate the exothermic combustion reactions that are then capable of establishing a self-sustaining, propagating reaction zone, sometimes referred to in the art as a "flame front". The remaining fuel mixture in the combustion chamber is ignited by the advancing flame front which moves radially outward at subsonic speed from the initiation region at the surface of the ignition kernel. Depending upon the turbulence condition within the combustion chamber and the laminar burn velocity of the mixture, the average effective flame front speed will usually be within the range of 15 to 30 meters per second.
The thermal criteria for plasma ignition kernel size, duration, and rate of expansion are generally based on the establishment of a temperature gradient, having, as a minimum, the same magnitude and spatial proportions as would exist in a self-sustaining reaction zone in the same mixture. This minimum temperature profile must then be maintained for the duration of the effective induction time of the combustion reaction sequence. The effective induction time decreases as the temperature gradient at the ignition kernel boundary increases beyond the minimum flame front requirements, thereby speeding up the ignition process.
On the other hand, the higher temperature gradients that accompany an over-driven kernel promote more rapid heat losses that, unless offset by ignition system energy delivery, lead to faster cooling of the plasma volume. This can result in a slowing down or termination (quenching) of the thermally driven ignition process. Simplified quantitative treatments of this process have generally been based on the balance between ignition system energy input in the form of plasma heating, and energy output in the form of thermal losses to the spark gap electrodes and the cooler surrounding gas mixture. Such thermal ignition models often view the plasma kernel as quasi-static, usually assume thermodynamic equilibrium, and neglect the rapid, dynamic breakdown processes that initially create and expand the discharge channel. Ignition models also usually neglect the detailed complexities of chemical combustion kinetics. Thermal models apply reasonably well to relatively long duration arc and glow discharge operation which is characteristic of conventional ignition systems.
Because of ignition delay associated with chemical reaction induction time, and due to the relatively slow propagation velocity of the combustion flame front, it is normally necessary to initiate the ignition spark in an IC engine well before the piston reaches top dead center (TDC) at the end of the compression stroke. This advance in ignition timing causes a portion of the fuel to be burned before the piston reaches TDC, thus resulting in negative work and loss of torque; this problem is exacerbated with slower burning, harder to ignite (longer induction time), leaner mixtures which demand greater timing advance.
With the foregoing basic principles of thermal ignition as background, recent approaches toward ignition enhancement have been directed at empirically optimizing spark ignition electrode geometry, orientation, and placement within the combustion chamber, as well as extending the duration and/or spatial distribution of the plasma kernel. Known ignition enhancement systems usually operate at higher energy levels, ranging from about 60 mJ to several joules per pulse. These systems may provide a single, long lasting glow or low current arc discharge, or a sequence of several shorter discharges which yield effective ignition kernel durations from 2 to 10 milliseconds. Greater spatial distribution of the kernel is most often achieved by using a wider discharge gap. This requires an ignition system capable of consistently delivering the higher voltage necessary to ensure gap breakdown.
Another approach to kernel distribution is the use of multiple ignitors at different locations in the cylinder head. Still other techniques have involved inducing plasma kernel motion through the application of electromagnetic body forces and thermal pressure, thereby propelling the kernel well into the fuel mixture and away from quenching surfaces. More particularly, the plasma jet ignition (PJI) has undergone considerable investigation during the last decade and has been shown to be very effective in promoting faster, leaner engine combustion. The PJI possesses excellent ignition probability characteristics even with ultra-lean fuel mixtures and is not prone to classical misfire. Furthermore, it has been shown to exert influence beyond the early ignition phase and to enhance later combustion by introducing turbulence effects and by distributing combustion promoting ionic species. Unfortunately, the plasma jet is undesirable from the standpoint of electrode erosion which renders it impractical for commercial use in engines.
Various other known experimental systems utilize laser, photochemical, and microwave techniques. However, none of these techniques have proven practical for commercial use.
The more practical engine enhancement systems of which we are aware have, with varying success, extended engine operation to leaner mixtures at the usual expense of highly advanced timing that is characteristic of slow, lower performance combustion. Better results are generally achieved in engines with fast burn chamber design, but stable, practical operation has rarely been extended to air-fuel mixture ratios significantly leaner than about 20:1 without suffering significant loss of engine performance, increased specific fuel consumption, and increased unburned hydrocarbon emissions.
Continued improvements in ignition enhancement systems have been limited by the traditional emphasis on establishing a thermally initiated burn kernel from an arc or glow discharge. These two relatively quasi-static modes of discharge operation are basically limited to low-power dissipation joule heating as the means of converting electrical energy into kinetic activation energy in the plasma ignition kernel. The resulting thermal kernel is mainly limited to the mechanism of gradient-driven heat flow as the means of transferring kinetic energy to, and inducing combustion in, the reactive mixture. This is augmented by the presence of reaction-promoting ionic species within the plasma. However, this overall process of energy conversion and transfer is accomplished in a relatively inefficient manner and, with few exceptions, is of insufficient intensity or too localized in influence to achieve far-reaching enhancement of the combustion process. Joule heating within the glow or arc phase results from discharge current power dissipation in an already established, highly conductive ionization channel. The power coupling efficiency from a relatively high impedance ignition source circuit to the very low impedance of an established discharge channel is quite low, resulting in a greater fraction of the available energy being lost through power dissipation in circuit resistance other than the discharge channel itself. Somewhat greater power dissipation in the discharge channel can be achieved by increasing the magnitude of current flow. However, for a given discharge duration, this may be accomplished only at the expense of greater energy input requirements and severe electrode wear.